Do some sanity check on the data for training a custom model

When creating a new custom model, the first step is to collect a corpus from the existing Nuxeo Database.
The first step should be a quick statistics collection of the data and give some feedback about the quality of the data, and, if possible, impossibility of training a model with it.

Feeds statistics into the UI to help the user create a model definition (docType, inputs, outputs)
Statistics supplied:

• Required for defining output fields
  • Cardinality
    • If possible with percentiles
  • Aggregation (bucketing/histogram)
    • Rules: score (green, yellow, red) and comment on values
    • Unbalanced data ?
    • Enough data for each value ?
    • All the same tag !
  • Required for defining input fields
    • Null fields
    • Total number of documents
    • Rule: score on possible use for training

Endpoint

• docType
• Inputs
• Outputs
• WHERE clause

Tasks

• Generic service
• Generic rest API
• Define rules for quality of output
• Define rules for quality of input
• Get statistics for input
• Get statistics for output